Speedcurve - Refresh

Tests scenarios have not been updated since DCR article migration days and hence many of the tests are not relevant anymore.

For concrete metrics like JS size, page weight, etc Speedcurve could still be useful.

For synthetic testing, there is still potential value in indicative changes over time.

Comparing ourselves to other orgs could be useful too:
https://app.speedcurve.com/benchmarks/

Investigate:
- what metrics we want to track
- what synthetic tests we want to run

Action:
- remove redundant tests and dashboards
- create new dashboards with desired metrics / tests